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Old Aug 29, 2012, 06:51 PM
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United States, NC, Raleigh
Joined Jul 2011
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Originally Posted by Ward Hagaman View Post
I wonder, is there a mixing/dispensing system that isn't too expensive to try? If you could spray the foam into the mold you might have more control over the application and more time.
I have a buddy that sprays two-part PU foam into wall cavities for a living, he knows EVERYTHING about this stuff. The expense of spray equipment is in it's ability to exactly meter the flow rate of the chemicals. Given the expansion ratio, measuring this stuff precisely while it's flowing to a spray gun is tricky business. The stuff he sprays has a sub-second mix time and goes off in two-seconds flat. Even so, properly metering the slower stuff would still be extremely challenging.

As you've discovered, the trick to getting it to stick well is to smear it around onto all surfaces before it goes off. When it goes, it forms a skin on any non-contacting surfaces. The skin doesn't stick well to anything, and you do have to be careful to provide venting forward of propigation. I suspect what's happening here (in your favor) is your allowing some gasses to escape through the perimeter opening (which is good).

Though it's likely you've got some voids in the center of the wing where the expanding (inward) skins met. This area will also not be very well stuck together, and be a much higher density then other areas. What my foam guru suggests is, do left/right wing-halfs, preferably open at BOTH ends if possible, and over-fill them by by some amount (you'll have to experiment).

The goal is to get the expansion going to such excess, that the inner-skins are forced out the end(s). You'll waste a lot of foam this way, however the result will be much more consistent density, and you'll have less problem with edge-voids (though smearing around thoroughly (like you are) is critically important.

I'm not sure about down-under, but here in the States they are coming out with slower-reacting low-density foams.. Granted it's probably a whole 10-20 seconds slower than yours, but every extra bit helps with this stuff. Good luck, and I'm glad you're brave enough to try this seriously, and really appreciate your sharing! Keep us updated!
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